Method for producing black coatings on metal surfaces



United States Patent 3,372,6l64 METHOD FOR PRGDUCING BLACK (IGATHNGS 0NMETAL SURFACES Robert R. Jones, Perry, and Howard G. Pekar, Cleveland,(Ellie, assignors to The Lubrizol Corporation, Wickliffe, Ohio, acorporation of Ohio No Drawing. Filed Jan. 6, 1967, Ser. No. 607,622filairns. (Cl. 148-615) ABSCT OF THE DISCLOSURE Black matte coatingswhich are uniform in appearance and thickness and stable underrelatively severe wear conditions are produced on metal surfaces bytreating the surface with an aqueous acidic solution of a trivalentantimony salt and then depositing a protective coating, preferably aphosphate coating, thereon. The coating is somewhat darker if thephosphate solution contains nickel, and the antimony-containing solutionremains homogeneous over longer periods of time if it contains a smallamount of a stannous salt.

This invention relates to coatings for metal surfaces and moreparticularly to a new method for producing a black matte finish on ametal object, which method comprises contacting the surface of saidobject with an aqueous acidic solution of a trivalent antimony salt andthereafter depositing a protective coating thereon.

The provision of a black coating of uniform appearance for metalarticles is of increasing importance, especially in the automobileindustry. Certain automobile manufacturers now require that partsfurnished tov them have such a coating. The dificulty has been thatuniform black coatings have not been capable of simple production. Themethods heretofore used have involved paints, pigmented oils oradditives to metal treatment baths (e.g., phosphating baths). Each ofthese is attended by serious disadvantages such as non-uniformity inappearance or coating thickness, smutting, evolution of flammablesolvent fumes, and low resistance to abrasion and Wear.

A principal object of the present invention, therefore, is to provide animproved method for producing black coatings on metal surfaces.

A further object is to provide a method for blackening metal surfaceswhich is simple to use and compatible with a wide variety of metaltreatment procedures.

A still further object is to produce black matte coatings on metalobjects, said coatings being uniform in appearance and thickness andstable under relatively severe wear conditions.

Other objects will in part be obvious and will in part appearhereinafter.

The method of this invention may be used with any metal which willreceive a phosphate coating. These include ferrous metals, cadmium, Zincand aluminum. The method is particularly advantageous on ferrous metalsurfaces.

As indicated above, the method of this invention involves an initialtreatment of the metal object being coated with an acidic solution of atrivalent antimony salt. Any of the water-soluble salts of trivalentantimony are satisfactory for this purpose; however, antimonytrichloride is preferred. Based on the chloride, the concentration ofantimony salt in the solution should be about 0.310.0 grams per liter,and preferably about 0.53.1 grams per liter.

The acid should be one which does not react violently or passivate thesurface of the metal being treated or oxidize the antimony salt. Thus,neither sulfuric or nitric acids are suitable for use, since each ofthese acids oxidizes trivalent to pentavalent antimony. Furthermore,nitric acid passivates the surface of steel. In general, a strongmineral acid is desired, and hydrochloric acid is generaly convenientand suitable. The concentration of hydrochloric acid in the solution ispreferably about 25-80 grams per liter. If lower acid percentages areused, insoluble antimony oxychloride may form and precipitate, While ifhigher percentages are used, uniformity of the black coating isimpaired.

The presence of a stannous compound in the antimony salt solution isoften beneficial, since stannous salts seem to retard the precipitationof antimony trioxide. About 0.30.4 gram of stannous chloride per gram ofantimony trichloride is sufficient to accomplish this purpose.

According to this invention, the metal object is first thoroughlycleaned and is then contacted with the antimonycontaining solution(e.g., by immersion or spraying) for a period of time sufficient toprovide a black coating of the desired thickness. The preferredthickness is in the range of about 20-70 mg. per square foot, preferablyabout 50 mg. per square foot. The time required to form a coating ofthis thickness is usually between about 10 seconds and about 4 minutes.For a 50 mg. per square foot coating, about 2 minutes is suitable. Thetemperature of the antimony treatment is preferably about roomtemperature (e.g., -80 F.).

Following treatment With the antimony solution, the metal object may berinsed with water or another suitable rinsing medium; this rinsing stepis desirable but not necessary. The object is then coated with aprotective coating.

Protective coatings suitable for use in the method of this invention-areof many types and include, for example, lacquers, plastic coatings,phosphate coatings and chromate conversion coatings. The phosphatecoatings are preferred; they are prepared by contacting the surface ofthe object with an aqueous acidic solution containing phosphate ions,usually in combination with auxiliary ions such as zinc, manganese,lithium, beryllium, magnesium, calcium, strontium, cadmium, barium andnickel. The presence of anions such as nitrate, nitrite, chlorate,ferrocyanide, ferricyanide or the like is also advantageous. Thecomposition of the phosphating solution is not critical, but excellentresults are obtained with the solutions described in US. Patent3,090,709 and in copending application Ser. No. 323,134, filed Nov. 12,1963. These solutions generally contain as essential ingredients thephosphate ion, either .zinc or nickel ion and at least one other metalnitrate from the group listed above. Zinc, if present, usually comprisesabout 0.1-0.4% (by weight) of the solution, and nickel may compriseabout 0.005 2.0% thereof. Nickel-containing solutions are especiallydesirable since the presence of nickel ion reduces the smutting tendencyof the blackened surface. The presence in the phosphating solution ofsmall amounts of antimony, due to drag-out from the pre-dip solutions,is not harmful and in some instances may increase the thickness of theblack coating.

The following table gives the compositions of several phosphatingsolutions which may be used'in the method of this invention. All figuresrepresent weight percentages of the various ions except for points totalacid, which denotes the number of milliters of 0.1 N aqueous sodiumhydroxide required to neutralize a 10 ml. sample of the solution, usingphenolphthalein as an indicator. The examples given are not to beconsidered as limiting the in- Patented Mar. 5, 1968' 4 vention, sinceany of the numerous phosphating composi- This concentrate is dilutedwith water to 3% (by voltions known in the art may be used. ume).

Example No. Ions The preparation of some of the phosphating solutions 15Example 12 in the above table is effected in the followingmanner.

A 3% (by volume) aqueous solution is prepared from a concentratecontaining 59.3 grams of the zinc oxide- T 24 6 f i lf 1 dd d 1 4 fnitric acid solution described in Example 11, 14.5 grams 0 grams 0 W1ere are a e grams 0 of 75% phosphoric acid, 16.9 grams of ammonium di-Iime Calcium Oxide) and grams of 75% hydrogen phosphate, 3.7 grams ofnickel nitrate hexamercial phosphoric acid. The solution is then dilutedby h d d 5,6 grams f water. s g; P i yf fl i) c l 1 p y Methods fortreating metal surfaces with phosphating 0 Water. 0 1 "r f 0 t 6 illteS0 utiOn t 6T6 is a ded solutions are well known in the art and need notbe degram of Sodium nitrite and 2 grams of nickel flitfati scribed indetail here. Suflice it to say that the phosphathexahydrate- 0 ingtreatment is generally carried out at an elevated tem- Example 2perature, usually about 150-210 F., and requires between about secondsand 2 minutes. Dipping, spraying or any other convenient means ofcontact may be used. 30 The blackened and phosphated metal surface ispreferably rinsed again with water and finally treated with an aqueouschromate or chromic acid solution, a so-called water-soluble oil, orboth. The latter term is generic to a number of complex metal salts ofmineralacid (especially phosphoric acid) esters, said salts beingsoluble To 87.3 grams of water there are added 11.7 grams of lime (72%calcium oxide), 68.4 grams of 75 commercial phosphoric acid, and 12.6grams of nickel nitrate hexahydrate. This concentrate, having a specificgravity of 1.3, is diluted by dissolving 2 parts (by volume) in 100parts (by volume) of water.

Example 3 30 or dispersible both in water and in organic solvents. Exhissolution is prepared by dissolving 2.3 grams of amples of water-solubleoils are the products described in lime (72% calcium oxide), 10.7 gramsof 75% commer- US. Patents 2,080,299; 2,820,723; 2,861,907; and 3,215,-cial phosphoric acid, 2.4 grams of nickel nitrate hexa- 715. Many ofthese are sold under the trademark Rusthydrate, 4.5 grams of 42 Baumnitric acid, and 10 4O arest. Either the chromate or water-soluble oiltreatment grams of ammonium dihydrogen phosphate in 970 grams rendersthe metal surface resistant to corrosion. of water. The effectiveness ofthe method of this invention is Example 4 shown by the followingexamples. To 830 grams of water there are added 121 grams of Example 13commercial phosphoric acid. 15 grams of Baum A panel of cold-rolledsteel 4" X 6") is cleaned, pickled mtric and, 8 grams of lime calciumOxide) and in 10% phosphoric acid for 4 minutes at 160 F., rinsed 15grams of mckel nitrate haxahydratewith water and immersed for about oneminute at 75 F. in an aqueous solution containing 0.75 gram per liter ofExample 5 antimony trichloride and 63.5 ml. per liter of 38% hydro- Anaqueous solution (1 liter) is prepared containing 00 acid (about grams Pliter f L PP- 4.98 grams of zinc nitrate, 6.88 grams of ammonium di-'proxlmately It 8 then rmsed with w hydrogen h h t d 6.32 grams of l initrate mersed for 15 minutes in the phosphate solutlon of Extrihydrate.ample 10 at 190 F., rinsed a third time and treated for 30 seconds at160.F. with an aqueous solution of chromic Example 6 acid (0.25 g./l.).The panel is finally air-dried, coated An aqueous solution (one liter)is prepared containing with a water-soluble oil sold as Rustarest14-40-SC and 14.2 grams of zinc nitrate hexahydrate, 7.8 grams of 75%air-dried again. The surface of the metal thus treated rephosphoricacid, 4.2 grams of zinc chloride, 8.7 grams ceives a uniform blackcoating.

of ammonium dihydrogen phosphate, and 14.3 grams of A second panel istreated as described above, except calcium nitrate trihydrate.

that the chromic acid treatment is replaced by a hot water rinse (20seconds at 180 F.). A third panel is treated Example 10 with theantimony-containing and phosphate solutions,

A concentrate is prepared from 31 grams of 75% Phos rinsed with water,immersed for 20 seconds at 160 F. 111

Phoric acid 263 grams of 67% nitric acid grams 3.20% (by volume) aqueoussolutlon of a water-soluble of Zinc Oxide, 120 grams of ammoniumdihyzirogen Phos 011 sold as Rustarest 452, and finally air-dried. Bothpanels phate, 3.7 grams of 50% sodium hydroxide, and 14.1 receive Sumlarblack coatmgs' grams of water. This concentrate is diluted with water toExample 14 3% (by volume). f l

- The procedure 0 Examp e 13 (with chromlc acid rinse Example H followedby air drying) is repeated; the antimony and A concentrate is preparedfrom 65.4 grams of an aqu phosphate baths are used continuously forcoating steel ous solution containing 20.95% (by weight) Zinc oxidepanels,with replenishment as needed. It is found that the and 31.2%nitric acid, 16.0 grams of phosphoric antimony bath must be recharged,after treatment of 24 acid, and 18.6 grams of ammonium dihydrogenphosphate. 75 and 36 square feet of steel, with 25 ml. of a concentratecontaining 42.8 grams per liter of SbCl and 500 ml. per liter of 38%HCl.

Example 15 A continuous operation similar to that of Example 14 isconducted, except that the antimony solution also contains 0.25 gram perliter of stannous chloride. It is found that the bath remainshomogeneous during the treatment of a considerably greater total surfacearea of metal.

Example 16 The procedure of Example 14 is repeated, using an antimonysolution containing 0.67 gram per liter of SbCl and about 43.4 grams perliter of HCl, and the phosphate bath of Example 11. A similar procedureis followed using the nickel-containing phosphate bath of Example 12.The latter is found to produce a somewhat darker coating.

What is claimed is:

1. A method for producing a black matte finish on a ferrous metal, zinc,cadmium or aluminum object which comprises contacting the surface ofsaid object with an aqueous acidic solution of a trivalent antimonysalt, the acid in said solution being one which does not react violentlywith or passivate the metal surface or oxidize said antimony salt, andthereafter depositing a phosphate conversion coating on said surface.

2. The method of claim 1 wherein the phosphate coating is applied bycontacting the metal surface with an aqueous acidic solution containingas essential ingredients the phosphate ion and one or both of zinc andnickel ions, the zinc comprising about 0.11.36% by weight of saidsolution and the nickel about 0.005-2.0% thereof.

3. The method of claim 2 wherein the phosphate solution contains nickelion.

4. The method of claim 2 wherein the metal surface is a ferrous metal.

5. The method of claim 4 wherein the antimony salt is antimonytrichloride and the acid used in the antimonycontaining solution ishydrochloric acid.

6. The method of claim 5 wherein the concentration of antimony salt inthe antimony-containing solution is about 03-100 grams per liter and theconcentration of HCl therein is about 25-80 grams per liter.

7. A method according to claim 6 which comprises (A) contacting thesurface of said object with said antimony salt solution: (B) thereaftercontacting said surface with an aqueous acidic solution containing asessential ingredients the phosphate ion and one or both of zinc andnickel ions, the zinc comprising about 0.11.36% by weight of saidsolution and the nickel about 0.0052.0% thereof; and (C) treating saidsurface with one or both of 1) an aqueous chromate or chromic acidsolution and (2) a complex metal salt of a phosphoric acid ester.

8. The method of claim 7 wherein the antimony trichloride solution alsocontains a stannous salt, about 0.3- 0.4 gram of said stannous salt,calculated as stannous chloride, being present per gram of antimonytrichloride.

9. A ferrous metal, zinc, cadmium or aluminum object carrying on itssurface a black matte coating applied by the method of claim 1.

10. A ferrous metal object carrying on its surface a black matte coatingapplied by the method of claim 4.

References Cited UNITED STATES PATENTS 1,43 6,729 11/ 1922 Scanlan etal. 1,493,012 5/1924 Abraham 1486.15 X 1,770,828 7/1930 Arent 117-1302,564,661 8/1951 Arent 117-130 OTHER REFERENCES Hopkins, The ScientificAmerican Cyclopedia of Formulas, 1925, Scientific American Pub. Co., pp.444, 448.

Meyer, Product Engineering, December 1943, pp. 792- 794, TJI, p. 93,148-6.14.

RALPH S. KENDALL, Primary Examiner.

